Cell wall

A cell wall is a rigid substance on the outside of certain cells, usually plant, fungi and bacteria cells.[1] Other components include cross linking glycans and pectins. Many proteins are found, largely those which act to remodel the cell wall polysaccharides, however highly glycosylated proline rich proteins are also found which are not enzymatically active. Plant cell walls are classified as primary and secondary. The primary cell wall is ellucidated below and is found in all plant cells. The secondary cell wall is found in tissues that require extra rigidity such as the stem, trunk, xylem or other specialised tissue. This secondary cell wall may consist of simply thicker cellulose microfibrils with more cross linking, or the deposition of lignin that makes up wood in the case of the xylem.

Cellulose microfibrils are initially synthesised by the cellulose synthase complex, a set of enzymes embedded in the cell membrane. The cellulose molecule is initially about 500 glucose molecules long. Neighbouring cellulose molecules interact via hydrogen bonding to form parallel, overlapping aggregates of around 40 molecules. These are referred to as a microfibril. These are then arranged in layers referred to as lamellae and bound together by cross linking glycans.

Cross linking glycans are highly branched polysaccharides that interact via hydrogen bonding with the cellulose microfibrils. These are composed of a backbone, usually glucose but also xylose or mannose with many side chains. These vary greatly between species.

Pectins are branched polysaccharides, typified by the huge numbers of negatively charged galacturonic acid residues. Owing to this negative charge, the addition of divalent cations, such as calcium allows these chains to be cross linked by the positive ions.